1. Field of Invention
The present invention relates to a process for preparing the copper film-plated (hereafter, called xe2x80x9ccopper-plated cordxe2x80x9d) cord and to a composition of rubber compound therefor. More particularly, the present invention relates to a process for preparing copper-plated cord of 20 to 90 nm of copper film for vehicle tire, by plating zinc or tin with strong tendency of ionization on the surface of steel cord and after drawing the zinc- or tin-plated cord, and then plating copper film onto it contacting to copper sulfate solution of 10 to 50 gram per liter.
2. Description of the Prior Art
Steel cords are inserted into the rubber compounds of the belt and carcass of a tire in order to sustain its heavy weight, to absorb properly impacts encountered during service, and to enhance its mechanical stability. Since steel cord does not adhere directly to a rubber compound, its surface has been plated with brass. Sulfur in the rubber compound reacts with copper of the brass forms a copper sulfate which is the most important material of the adhesion interphase, resulting a strong and stable adhesion between rubber bulk and steel cord. However, several reactions between brass and oxygen or coater are simultaneously carried out during curing, and thus, the adhesion interphase contains oxides and hydroxide of copper and zinc as well as copper sulfide. Adhesion is very important technology for manufacturing tire: a good adhesion between steel cord and rubber compound is essential for cord to keep the performance of tire from the impact during service. On the other hand, the adhesion is not easy, because their physical propertiesxe2x80x94rubber compound and steel cordxe2x80x94are markedly different.
Steel cord has been used for 50 years as a structure supporting material of tire, and its physical property steadily improved. Nowadays, high-tensile steel cord has been replaced by super-tensile one. However, the replacement of normal tensile one by high-tensile one is not exceeded over 10 years. On the contrary, brass has been consistently used as a plating material of steel cord because of its excellent processability and adhesion property. But the additional growth of copper sulfide and zinc oxide at the adhesion interphase during tire service, causes adhesion degradation. The heat generated from a tire during service and the contact with moisture and oxygen in the air accelerate the additional growth, bringing about inevitably adhesion degradation. In order to maintain an excellent adhesion property for a long service life of a tire, the copper content in brass and the plating weight of brass should be optimized in the manufacturing of brass-plated steel cord. At present, the copper content of brass is lowered to 63% and the plating weight of brass is also lowered to the level of 3.6 g per 1 kg of a steel cord. These changes aim to prevent excessive growths of copper sulfide and zinc oxide during service of tire by reducing the source materials of copper and zinc. The lowering of copper in the brass is also effective to reduce the reactivity of copper in the formation of copper sulfide.
Moreover, if the layer is thin during the formation of the brass layer by electroplating, the amount of the brass working as a solid lubricant reduces, thereby giving the bad effect to the wire break and surface roughness. The xcex2-structured brass is formed when the Cu contents of brass layer is low. The xcex2-structured brass has bad drawing property than xcex1-structured brass, which gives bad effect to the cold drawing of wire. Thus, it is very difficult to low the copper content in brass layer of tire cord.
In addition, the composition of the rubber compound has been optimized to obtain a strong and stable adhesion: the cure rate of rubber is controlled regarding the formation rate of copper sulfide and diffusion rate of sulfur in bulk rubber. However, the improvement of the adhesion between rubber compound and brass-plated steel cord has not been satisfied, because the additional growth of the adhesion interphase is essentially inevitable. The further reduction of the plating weight of the brass is impossible due to the increase in the roughness of the brass surface generated at drawing step of the cord, because the bare surface of iron requires more work, producing non-even surface. Uneven plating of the brass causes heterogeneity in copper content, which brings about the insufficient growth of copper sulfide and poor adhesion.
In order to increase the adhesion stability of steel cord and rubber compound, the type and quantity of the additives such as the sulfur (vulcanizing agent), vulcanization promoter, adhesion promoter and filler must be optimized. Furthermore the condition of the vulcanization such as time and temperature, and the composition and thickness of the brass layer must be adjusted precisely.
Another attempt to enhance the adhesion property is by changing the order of plating material or adding the Co or Ni in the brass layer. And another way is to plate another material between substrate and brass layer, but productivity problems prevent the mass production of such products.
But the serious problem with brass as a plating material of the steel cord is overgrowths of copper sulfide and zinc oxide. Although copper sulfide is an essential material of an adhesive interphase and zinc oxide is helpful to control the formation rate of copper sulfide, excess growth brings about severe degradation of the adhesion interphase. The copper and zinc plated on the steel cord are heated by running tire and the heat accelerates reaction with the humidity and oxygen in the air, resulting in the further growth of copper sulfide and the further loss of metallic zinc. Since the contact between tire and humidity or air is inevitable, the adhesion degradation is not fundamentally inhibited when the brass is used as a plating material.
It is possible to use the brass-plated steel cord to enhance the stability of a tire by optimizing the brass composition and brass-plating weight, and by designing a proper rubber compound for it, however, these methods are limited in retarding the adhesion degradation. Since almost all of heavy duty tires such as truck and bus tires are reused by retreading them up to several times to save material waste, the reinforcement of tire structure by steel cord and the adhesion stability between the rubber compound and steel cord is considered more significant in order to enhance tire endurance. Accordingly, it is highly desireable to develop substitutes for brass as a plating material, which maintain adhesion interphase integrity strongly under severe service conditions of a tire for a long period of time.
Copper sulfide formed between the brass-plated steel cord and rubber compound acts as an adhesive, providing a strong and stable adhesion. On the contrary, copper sulfide formed between the copper plate and rubber compound does not exhibit any adhesive role. Overgrown copper sulfide is easily separated from the copper plate and then attaches to the rubber side, because the mechanical property of copper sulfide itself is very weak. However, it is previously known that an ultra-thin copper film onto a steel plate provides strong adhesion with a rubber compound. The obstacle to the application of copper film as an adhesive material is the difficulty in the commercial manufacture of the copper-plated cord with ultra-thin copper film. The exposure of a small part of bare iron and heterogeneous plating of copper causes a serious degradation in adhesion, thus, the use of copper as a plating material is not performed.
Plating an ultra-thin copper film on steel cord instead of a brass has several advantages sufficient formation of adhesion interphase, suppressing overgrowth, reduction of manufacturing time of a tire, and enhancement of the stability against the moisture and long-term storage due to the copper""s better stability to moisture than brass. Furthermore sustaining adhesion interphase from dezincfication during salt solution aging is expected, because of absence of zinc element. However the advantages of copper-plated cords are diminished with uneven thickness of copper film due to the roughness of the steel rod; i.e., in parts where the copper is plated too thick, the adhesion will be poor because of the overgrowth of copper sulfide, and in parts where the copper is plated too thin, the exposed steel surface will lead to an easy rupture in the adhesion phase.
A process is disclosed for preparing a copper-film-plated steel filament comprising plating zinc or tin on the surface of a steel filament, drawing the zinc or tin-plated steel filament, and contacting the drawn zinc or tin-plated filament with a solution of cupric sulfate, cupric nitrate, cupric chloride, or cupric acetate to plate a copper film on the surface of the zinc or tin-plated steel filament. The copper-film-plated steel filament produced by the process according to the present invention comprises a steel filament, a layer of zinc or tin film on the surface of the steel filament, and a layer of a copper film on the surface of the layer of zinc or tin film. The copper-film-plated steel filaments according to the present invention are used to form steel cords for use in rubber articles such as tires.
It is an object of the invention to provide a process for preparing a copper-film-plated steel filament by plating copper film on the surface of the zinc or tin-plated steel filament to form a copper-film-plated steel filament. It is another object of the invention to provide a process of preparing steel cord suitable for use in tires by twisting a plurality of the copper-film-plated asteel filament together. It is a further object of the invention to provide rubber articles such as tires wherein the rubber articles contain steel cords comprising a plurality of copper-film-plated steel filaments according to the present invention.
Compared with the use of brass-plated steel cord, manufacturing tires with copper-plated cord according to the present invention reduces manufacturing time due to faster formation of adhesion interphase, increases the storage period by enhancing moisture stability, and retards adhesion degradation thereby extending the service life of the tires.
In the process of the invention, metallic zinc or tin is plated on the surface of a steel filament and the zinc or tin-plated filament is contacted with a solution of cupric sulfate, cupric nitrate, cupric chloride, or cupric acetate to plate a copper film on the surface of the zinc or tin-plated steel filament to form a copper-film plated steel filament having an outer layer of copper film. The concentration of cupric salt in the plating solution ranges from 10 to 50 gram per liter. The plating process is preferably carried out with vigorous circulation of the plating solution in the plating bath. The thickness of the zinc or tin plated on the steel filament ranges from 0.05 to 2.5 xcexcm. The thickness of the copper film plated on the zinc or tin-plated steel filament ranges from 20 to 90 nm. The thickness of the copper film plated on the zinc or tin-plated steel filament preferably ranges from 30 to 70 nm.
The copper-film-plated steel filament may also be prepared by plating zinc or tin on the surface of a steel filament, drawing the zinc or tin-plated steel filament, twisting the zinc or tin plated filament, and contacting the drawn filament with a solution of cupric sulfate, cupric nitrate, cupric chloride, or cupric acetate to plate a copper film on the surface of the zinc or tin- plated steel filament.
The process of the present invention produces copper-film-plated steel filaments that are adherable to rubber. Steel cords suitable for use as tire reinforcing elements are produced from a plurality of copper-film-plated steel filaments. The thus produced steel cords are used in rubber articles such as tires. The rubber articles may additionally contain cobalt salt.